Hello Sunflower, I'm sorry for you this is your first post. Are you sure you want to spend time & energy investing in someone you can't trust. Someone who has lied to you over and over. Being engaged is supposed to be some of the happiest days ever. Not consumed by worry of what he might be doing, but what he IS doing and then lying to you about.

Don't know if you are in America but if so him carrying around pictures of young girls in skimpy, sexy clothing is dang close to being child porn if not child porn altogether. Do you suspect he might have full on child porn hidden somewhere since he knows you are so opposed to it?? Might be worth checking before taking on responsibility of marriage.

Having a child is a gift, they pick up on tension & worry even at his age. Not fair, as he can't help the choices & decisions his father makes.
Your post is very alarming to me on many fronts. The welfare of you and your innocent child, deception in your relationship because of the nature of it. The
possible legal ramifications, just the "unknowns" surrounding his behavior.
You deserve so much better for you & your son. My advice is RUN and fast!!
Visitation with his dad can be worked out later. Get your ducks in a row & leave this relationship, wherever or whatever it takes. Look up statistics of likelihood that he'll change based on what you "know" about his obsession with porn especially child porn.
Again so sorry you are faced with this when you should be so happy & carefree.

Only wish the best for you.